1. Field of the Invention
The invention relates to the field of memory accessing, particularly memory accessing with a limited address space.
2. Prior Art
Many methods are known for using address signals from a processor to access memory. In some of these methods, a limited address space is used to access memory having a greater capacity than that defined by the address space. One example of this is bank switching; this permits the limited address space to access two or more banks of memory. The processor under program control selects an appropriate bank, for instance, by enabling one memory bank rather than the other.
The present invention provides an apparatus for permitting an integrated circuit read-only memory (ROM) to be programmed to operate in one of two addressing modes. In one mode the memory operates as an ordinary ROM. In the other mode not all of the address lines are used, thereby freeing address space to access other memory. In effect, in this mode paging occurs within the ROM to permit access to the entire contents of the ROM. Importantly, this is accomplished without additional pins or terminals on the ROM.